Mechanical actuating means, hereinafter "actuators", have been used prior to this invention to removably mount and to actuate vented pipettes so that the contents are dispensed. By the push of a lever or a button, a ratchet is advanced in steps to cause a piston or plunger of the pipette to repeatedly dispense a predetermined, fixed amount of liquid. The advantage of such a mechanism is that it reduces the opportunity for the errors that often occur when pipettes are operated manually. When a new pipette is filled for dispensing, the previously emptied pipette is removed and the new one mounted in the actuator. Examples of such devices are described in U.S. Patent No. 3,161,323.
One difficulty with such mechanical actuators is that, unless prohibitively expensive pipette designs are used, the piston of the pipette is not reliably at a predetermined height as to accurately line up with the driving surface of the actuator. Because of this, either the actuator or the pipette must be adjusted by less than a whole step to bring the actuator's driving surface into contact with the piston of the pipette, a process hereinafter referred to as "initializing." However, initializing by altering the location of the driving surface is generally not desirable. It is particularly unsatisfactory when the actuator is programmed to operate in fixed steps only, as noted. If the pipette piston's initial location should be between one of the fixed levels defining the steps, the driving surface would have to be moved downward or upward during initializing by less than a whole step. Such movement would alter the volume of the first quantity to be dispensed.
On the other hand, initializing by adjusting the pipette and particularly the piston of the pipette has not been desirable prior to this invention. At best, manually readjusting the pipette to obtain contact with the driving surface is time consuming. At worst, such adjustment might require that the piston be lowered to the level of the driving surface. If the vent of the pipette accidentally becomes closed while the piston is being so lowered, undesired liquid dispensing occurs. Since the pipette vent is conveniently an aperture through the top of the piston at the very place force is normally applied to depress the piston, it has been difficult to downwardly adjust the piston without closing the vent.
Because of the above problems, use of mechanical actuators of pipettes has been less than satisfactory. Particularly the initializing problem has been acute when many individual pipettes are to be sequentially inserted into the actuator, each with a potentially different initial piston location.
Therefore, what has been needed is a mechanical actuator for a pipette and particularly a vented pipette, which automatically orients the piston and actuator driving surface without any loss of the liquid contained in the pipette.